Tokkai 6-101754 published by the Japanese Patent Office in 1995 discloses a non-finite speed change ratio continuously variable transmission comprising a continuously variable transmission and a planetary gear mechanism, for enlarging a speed change region of the continuously variable transmission (CVT) of a vehicle.
In this device, an engine output is input to a toroidal type continuously variable transmission and a reduction gear unit.
The output shaft of the continuously variable transmission is joined to a sun gear of the planetary gear mechanism, and the output shaft of the reduction gear unit is joined to a carrier of planet gears of the planetary gear mechanism via a power circulation clutch. A ring gear of the planetary gear mechanism is joined to a drive shaft driving the vehicle wheels, and the output shaft of the continuously variable transmission is also joined to the drive shaft through a direct clutch.
This device has two power transmission modes, i.e. a direct drive mode in which the direct clutch is connected and the power circulation clutch is disconnected, and a complex mode in which the direct clutch is connected and the power circulation clutch is disconnected.
In the direct mode, the engine output is transmitted to the drive shaft via the continuously variable transmission.
In the complex mode, a rotation of the ring gear stops when the sun gear which is rotated by the output of the reduction gear unit and the rotation of the planet carrier which is rotated by the output of the continuously variable transmission, are balanced. This state is a "neutral state" wherein a rotation torque is not transmitted to the ring gear and the drive shaft connected thereto.
In this case, the ratio of the engine rotation speed and the rotation speed of the drive shaft is infinity.
If the speed change ratio of the continuously variable transmission in this state is a neutral speed change ratio, the rotation direction of the ring gear varies depending on whether the speed change ratio of the continuously variable transmission is larger or smaller than the neutral speed change ratio. The vehicle is made to move forwards or reverse from rest by making the speed change ratio of the continuously variable transmission vary towards larger or smaller from the neutral speed change ratio.
When the vehicle is moving forwards and the speed change ratio of the continuously variable transmission arrives at a predetermined region, the power circulation clutch is cut off and the direct clutch is connected. Due to this, a change-over occurs from the complex drive mode to the direct drive mode. When the speed change ratio is at a higher speed than this region, in the direct drive mode, the output of the continuously variable transmission is directly transmitted to the drive shaft.
Therefore according to this speed change-over device, a change-over between all running states including forward and reverse from rest and high speed can be made smoothly without using a torque converter.
The speed change ratio of a toroidal type continuously variable transmission is made to vary by displacing trunnions which support power rollers due to oil pressure. More specifically, an oil pressure is applied to both ends of a piston joined to each of the shafts of the trunnions, and the trunnions are displaced due to this differential pressure. This differential pressure is controlled by a solenoid which responds to a duty signal transmitted by a control unit, and a spool valve which responds to a signal pressure generated according to the energization of the solenoid.
The differential pressure acting on the piston is reversed when the direction of travel of the vehicle is reversed, and this also occurs when there is a change over between the complex mode and the direct mode.
This reversal of differential pressure requires a large variation of oil pressure and makes oil pressure control complicated. Also, the duty ratio of the oil pressure control signal needs to be maintained between forward and reverse in order to maintain a neutral state. At such a duty ratio, oil pressure vibration may occur. This oil pressure vibration causes the output shaft of the transmission to vibrate, and is a factor responsible for generating noise.
Further, when there is a scatter in the signal pressure generated due to energization of the solenoid, it causes scatter in the differential pressure. Due to this scatter, the trunnion may be maintained in a biased position either to the forward or reverse side of the completely neutral state, and a small forward or reverse drive force may therefore act on the vehicle.
As this drive force is small, it can be controlled by operating the brakes, but it still gives the driver an uncomfortable sensation.